Last year, I got an extraordinary break. Tom
Waineo, a retired optician, offered to
grind me a mirror. You may remember Tom Waineo as the creator of the desktop polisher,
featured in last year's Sky & Telescope.

Anyway, Mr. Waineo said he had a badly chipped 2" Cervit blank. For not more than
cost, he could turn that glass into a quality 24" mirror. Well, I jumped at the
chance!

With a 104.5" focal length, I figured the resulting dobsonian telescope would be
about 8' long. I little long for my 12'x14' observatory, but who cares? For 24 inches of
glass, I'd make it fit!

The first step was to figure out the size of the diagonal. I wanted the diagonal to
work for both visual and photography. Some people use two diagonals, but I didn't want to
fiddle around with expensive glass all the time.

Fortunately, I already created an Excel spreadsheet to compute airy circle sizes and
Strel ratios. After playing with the numbers, a 4.25" diagonal gave a .93" Strel
ratio and a 1.03" airy circle. 35mm film has a 1"-by-1.25" area, so a
1" square area should expose most of the film. I was going to buy a 4.25"
diagonal, but Mr. Waineo talked me into letting him grind it.

The next step was to order a 24" truss tube kit from Astrosystems. With this kit,
you assemble the mirror box and rocker box. You can assemble the focuser cage or have
Astrosystems do it. To save time and effort, I decided to order the pre-assembled cage.

The owner of Astrosystems wrote a nice computer program. Punch in the numbers, and you
get a printout of the dimensions of all the 3/4" wood pieces you would need for the
bottom assembly, rocker box, and ground plate.

In addition to the printout, Astrosystems sent me a book illustrating these
calculations. Every possible weight and measurement was taken into account. To change the
balance of this scope, you simply change the length of the mirror box: a longer mirror box
for a heavier top assembly, and a shorter box for a lighter front end.

Astrosystems suggested using aircraft plywood, but the nearest dealer was an hour drive
away. So I settled on A-A cabinet grade plywood.

I cut out small pieces of paper (1/8" to the inch) representing all the 3/4"
panels I would need. I arranged these pieces until they could fit on two sheets of plywood
(don't forget to include an extra 3/8" for the cutting blade).

The next step was to head for the lumber company, buy my two sheets, and have them cut
it up. Very quick and neat. I talked a friend into jigger sawing the hole needed for the
top of the mirror box.

Next, I glued and screwed my side panels to the rocker box, making solid 1.5" side
panels. Then came the hard part. Cutting the altitude circles. The 18" altitude
wheels, provided in the kit, have to sit 9 inches above the bottom of the rocker box. The
center point would thus sit well above the top of the rocker box. So I butted the two side
panels together and tacked my drawing stick on each panel so I could draw circles on the
opposite panels. Then I cut the circles out with a jigger saw.

After rough sanding all the pieces, I glued and screwed the mirror box together. For
added strength, I also screwed pieces of 2" by 2" on the inside corners.

Assembling the rocker box took slightly longer. For some reason my front and back
pieces were a little short. Turned out that the bottom plate was about 3/4" too wide.
Still enough lip for the wagon wheels, but something you need to keep an eye on.

I glued my back-ebony Formica to the bottom of the rocker box. I drilled out two holes
at the top of the mirror box for two of the truss tubes. I drilled and mounted the Teflon
pads on the ground plate. I also drilled out the center hole to the rocker box.

Now came the wood dope. Unlike aircraft plywood, cabinet plywood has gaps between the
glued sheets. Every gap had to be filled. I also doped over all the screw heads. Then I
had to sand down the excess dope off. I had to repeat this process a couple of times.

In all this sanding, I found another reason to use aircraft plywood. Cabinet wood is
only a 5 ply, with a very thin outer ply. I quickly found that by sanding too deep or
sanding too much off a corner exposed the dark secondary ply.

Now was a good time to paint the inside of the mirror box black. I figured what little
I'd spilled could be sanded off. I applied two coats of interior flat black. Careful out
to finger this surface, as fingerprints tend to stick out.

Thank goodness for stain. After another sanding with a finer grain paper, I gave
everything two coats of Mini-Wax 'Golden Oak'. After applying each coat, I let the wood
sit a couple of minutes before wiping the excess off. This stain did a real good job of
hiding wood dope and other 'mistakes'.

For a finish coat I got some Helmsman Spar-Erathyane clear semi-gloss. A good
indoor-outdoor paint. It should stand up to dew. The first coat I laid on thick. The
following coat raised the grain, requiring a fine sanding. I had to coat and sand a couple
of times, until the wood was smooth to the touch.

All I had left to do was to mount the truss tube blocks and the altitude wheels. But I
didn't have the mirrors yet. So with winter approaching, final assembly had to wait for
better weather.

While waiting for spring, I ordered an equatorial platform from Tom Osypowski. This
platform has dual axis control, allowing prime focus photography for up to an hour at a
time.

When spring finally arrived, I had everything I needed for final assembly. I got help
and mounted the truss tube blocks to the mirror box. Then, with help from a fellow society
member, I assembled the telescope at our society's observatory. We used a couple of
C-clamps to hold the altitude wheels, while sighting on a radio tower. After some
adjustments, we found the correct balance point for the wheels.

I didn't waste any time getting my telescope up to my observatory. It took two people,
but we managed to assemble the 24" telescope.

Collimation wasn't easy! Before final assembly, I took a lazy suzan and spun the main
mirror to mark the center. I also used a template to mark the center of the diagonal,
offset and all. Once assembled, the next step was to get the focuser cage centered. So I
had to get the mirror box perfectly leveled. I adjusted the truss tubes until the cage was
leveled. Then I used a plumb to make sure the diagonal holder was centered to the main
mirror.

Probably the most useful tool I ever purchased was a laser collimator. Once I got the
focuser cage centered, it took only 10 minutes to collimate the telescope. I already saw
first light during the last full moon. I couldn't see faint objects, but at least my airy
disks are round and centered.

Oh sure, I made many mistakes building this telescope. Perhaps the biggest was using a
jigger saw to cut my rocker box circles. The first side the cut was great. But on the
other side, the blade got away from me. A real bad inside angle. By the time I finished
sanding everything even, I lost a good 1/4" and both sides. And still not totally
perpendicular. Next time, you can bet I'll use a band saw.

Another mistake was in the front of the rocker box. For some reason, the front panel
was more than five inches too high! It didn't allow the fully assembled telescope to be
positioned horizontally. I couldn't close my observatory until I had cut notches in this
panel to make room for the lowest truss tubes. Had I checked the fully assembled telescope
beforehand (Hint: HST), I could have corrected the problem quicker and easier.

But I lucked out, too. The balance point for the altitude wheels turned out to be 2
inches low. I had to notch the rocker box for my two front truss tubes, but so what? That
raised the mirror box just enough to clear my azimuth bolt and computer encoder.

So far I'm happy with what I got. I wouldn't, however, consider this telescope
portable. In fact, I plan to adjust the telescope for photography and use an eyepiece
extension tube for visual use. The truss tubes are just too difficult to adjust
frequently.

1996 Events

Well, I finally had some real chances to use the telescope. So far, the telescope is
meeting all expectations. The visual images are real good! At first, however, I was
awfully disappointed with views of M31. Then I found the problem. You see, at this time of
year this object rises very late. By 3 o'clock in the morning, my diagonal was wet with
dew. So I bought a 12V hair dryer from Orion. Now I can see two dust lanes in M31, with a
hint of a third.

Dew was also getting to my primary mirror. So I had the local awning company sew me a
black shroud (telescope cover). It works! The shroud keeps the dew out, keeps stray light
out, and helps steady the heat turbulence.

And just as I was breaking the telescope in, guess what? My equatorial platform
arrived. It took two people to install the platform. Runs like a dream. Easy to
polar-align and can run for over an hour.

Found out one problem (or feature) of the platform. When I move the telescope, if the
platform is already rising (last 30 minutes), it tends to slide back down. In other words,
if you move the telescope around all night, you may only need to reset it a couple of
times. But this slipping, however, goofs up the NGC-MAX computer. So to use the computer,
I have to install the shipping block. Which means I have to turn the platform off.

Though I haven't used it for an entire year, I'm optimistic. I've already taken prime
focus pictures and wow! Do they look fantastic! Can't wait for Orion to rise. I've always
want to do a good horsehead!

2002 Events

Well, I've had my telescope a couple of years and couldn't be happier. I've taken fantastic prime focus and piggyback pictures, which can be seen on
this web site.

Had a few problems, very minor. The platform was still slipping, goofing up the NGC-MAX. So I took the telescope apart, cleaned it up and gave the Teflon pads a coat of car wax. The telescope moves a lot easier now. Still a little top heavy, but now I can use the NGC-MAX. Remember not to put on TOO much wax. Oh, I had to remount my computer's altitude encoder. It kept slipping.
I mounted it to the arm instead of the wagon wheel.

The mice kept chewing on my telescope shroud, so now I store everything in a big self-sealing
tote. Otherwise, everything is fine. My only desire now is to get the main mirror recoated. The current coat is ok, but you shouldn't be able to shine a flashlight through it.

2014 Events

After almost twenty years of use, my big scope is still doing great. Went to
using a Canon Rebel XT, then a Rebel XSi, which has a live view feature. Prime
focus shots
that would take a half hour with film I can now do in five minutes. Amazing!

After reading an article about cleaning mirrors in S&T, I decided to
clean my 24Ē mirror. Started off with distilled water and a little bit of dish soap. Used surgical cotton to roll and
lift the dirt out. I tried to be careful, but couldnít get out several dirt specks and ended up putting a couple
small streaks in the mirror. Anyway, I rinsed the mirror with more distilled water. As the mirror dried I used a
can of air to blow off the water beads that were forming (donít spray all at once or youíll get
cold liquid out of the can). Despite the streaks and dirt specks, my mirror hasnít been this clean in years.
I accidentally washed off my center dot. My mirror has a thin coat, however, so
I can see the laser beam on the back side. Was easy enough to make a new center
mark on the back side.

Note: One thing I didn't use was Isopropyl Alcohol. I understand why you want to use
it, so that it evaporates completely. But even todayís 99% stuff will leave a residue.
Not worth it.

2016 Events

When the local astronomy club rebuilt their big 16'x20'
roll-off roof, the original plan was to move the club 15" telescope
into it. It became clear, however, the 15" telescope was going to need a lot of
work. So the club decided to mothball the 15" telescope for now and let me to
put my 24" telescope in the big roll-off. As it turned out, well worth the
move. The big roll-off is ideal for this telescope. The walls are far enough away
that the telescope never touches them. No need for the telescope to sit on two layers of
bricks, so even looking straight up is comfortable on an eight foot ladder. With my
big telescope closer to home, I've already used it more times this summer than the
last couple years up north. Images are fantastic, at least that's what the club
members tell me (might have to install a coin operated device so I can get a
turn)<g>

2017 Events

At last year's move, my big telescope cleaned up pretty good. Even got a chance
to spin the main mirror and put down a new center marker. But after twenty
years of mice nests, the top of the mirror box got soiled. This summer I sanded
the top of the box and actually got much of the mice stain out. Then I put down
a new coat of stain and two fresh coats of Spar-Erathyane. Not perfect, but not
bad.

This summer I also spent time aligning the platform. By the end of summer, I
was taking 90 second unguided shots. 90 seconds! I talked to Tom Osypowski of equatorial platform.
He was shocked as well! I told him my south alignment star wasn't moving, but my
east alignment star had a small right ascension drift. Tom suspects that drift
must be caused by weight unbalance. Instead of messing with the alignment, Tom
suggested I get a better camera. Something like a Canon T6, that would allow me
to set a higher ISO and thus a shorter exposure.